1. Field of the Invention
This invention relates to a catalyst for the decomposition of nitrogen oxides and a method for purifying a diesel engine exhaust gas by the use of the catalyst. More particularly, it relates to a catalyst which is capable of decomposing and allaying particularly nitrogen oxides (NO.sub.x) among other harmful components in the diesel engine exhaust gas and, at the same time, depriving by combustion the exhaust gas of carbonaceous particulates, unburned hydrocarbons, and carbon monoxide and a method for purifying the diesel engine exhaust gas by the use of this catalyst.
2. Description of the Prior Art
The nitrogen oxides (hereinafter referred to as "NO.sub.x ") in the diesel engine exhaust gas cause photochemical smog and acidic rain. In recent years, since the emission of the NO.sub.x from the diesel engines has been posing a social issue particularly in the urban districts, the alleviation of this emission is an important consideration. For this reason, studies aimed at a catalyst for the disposal of the exhaust gas are now under way. Further, since the diesel engine exhaust gas contains particulates harmful to the physical health, the catalyst for the disposal of the exhaust gas of this nature is naturally expected to be capable of decomposing the NO.sub.x and repressing these particulates as well.
It has been customary heretofore to use a three way catalyst for the purpose of purifying the exhaust gas from an automobile. Since the exhaust gas from a diesel engine excessively contains oxygen, however, the ordinary three way catalyst is incapable of thoroughly allaying the NO.sub.x .
A catalyst having copper deposited on a porous carrier such as of zeolite, alumina, or silica as disclosed in JP-A-63-100,919, for example, has been available for the removal of the NO.sub.x from the exhaust gas of a diesel engine or the exhaust gas of a gasoline-lean burn engine which contains oxygen in a large quantity. This catalyst, however, is at a disadvantage in being deficient in resistance to heat and liable to be poisoned by sulfur oxides.
A method for removing the NO.sub.x by using platinum as a main catalyst in the presence of sulfur oxides has been disclosed, for example, in JP-A-05-137,963. This platinum containing catalyst naturally is highly active in oxidizing SO.sub.2. When it is used in the disposal of the diesel engine exhaust gas, therefore, it increases sulfate group owing to the oxidation of SO.sub.2 and possibly increases particulates rather than decreases them.
As remarked above, the catalysts heretofore proposed for the removal of nitrogen oxides still have a problem from the practical point of view for the purpose of purifying the exhaust gas from a diesel engine.
It is an object of this invention, therefore, to provide a novel catalyst for the decomposition of nitrogen oxides and a method for purifying the diesel engine exhaust gas by the use of this catalyst.
Another object of this invention is to provide a catalyst which is enabled by the addition of a reducing agent to attain efficient decomposition of the NO.sub.x in the diesel engine exhaust gas and a method for the use of this catalyst.
Still another object of this invention is to provide a catalyst which effects removal by combustion of such harmful components as unburned hydrocarbons and carbon monoxide, allays particulates, and possesses an ability to withstand elevated temperatures.